Films made from aliphatic polyester, inclusive of polylactic acid, have been developed to acquire natural decomposability upon disposal in a natural environment. For example, a polylactic acid film is naturally hydrolyzed in soil and decomposed into harmless substances with the aid of microorganisms.
On the contrary, the preferential aim of acquiring natural decomposability has prevented sufficient expression of the superior properties that aliphatic polyester inherently has. To be specific, advanced orientation and crystallization of the film inhibits hydrolysis and decomposition by microorganisms. To suppress orientation and crystallization of the film, therefore, strength and thermal dimensional stability have been sacrificed to a certain degree.
In the meantime, attempts have been made to improve degree of orientation and crystallinity of the film, thereby to increase strength and thermal dimensional stability. For example, JP-A-7-207041 discloses polylactic acid made from a polylactic acid polymer, which has a plane orientation .DELTA.P of not less than 3.0.times.10.sup.-3, and which shows a difference value (.DELTA.Hm-.DELTA.Hc) between the amount of crystallization melting heat .DELTA.Hm upon elevation of the temperature of the film, and the amount of crystallization heat .DELTA.Hc generated by crystallization during temperature elevation of not less than 20 J/g.
This polylactic acid film does not aim at improving hand tearability, twist retention and post-processing stability, such as printability, required of a packaging film. Moreover, the polylactic acid polymer film exemplified therein has insufficient properties, particularly post-processing stability, such as dimensional resistance at 120.degree. C. necessary for printing, laminating and the like, such that this film is insufficient to achieve the above-mentioned goal. In the prior art technique relating to other aliphatic polyester films, no attention has been paid to the reduction of viscosity or properties of polylactic acid polymer even when attempting to improve hand tearability, twist retention and post-processing stability, such as printability, of a packaging film.
Conventionally, polyethylene terephthalate oriented films have been used for packaging bags, adhesive tapes and the like because of the superior properties they have in toughness, heat resistance, water resistance, transparency and the like. In general terms, a packaging film for fresh food, processed food, pharmaceutical products, medical equipment, electronic parts and the like should permit easy tearing with hand, i.e. "hand tearability", and also should be able to stay twisted upon twisting, i.e. "twist retention". Polyethylene terephthalate oriented films have the above-mentioned superior properties but they are poor in hand tearability. As a result, the film causes difficulty in tear opening a packaging bag or cutting an adhesive tape. Further, the film is defective in that it cannot be used for twist packaging due to its poor twist retention.
A film having superior hand tearability is cellophane. It is superior not only in hand tearability but also in properties such as transparency, twist retention and the like, and is very useful for various packaging materials and adhesive tapes. However, cellophane shows moisture-adsorption characteristic, which prevents provision of constant quality of the film due to seasonal variations in these properties.
As a film that satisfies hand tearability and twist retention, for example, a uniaxially oriented polyester film (JP-B-55-8551) and a polyethylene terephthalate film (JP-B-56-50692) obtained by copolymerization of diethylene glycol component and the like, have been proposed.
The above-mentioned uniaxially oriented polyester film permits linear splitting in the orientation direction but otherwise in other directions. The polyethylene terephthalate film obtained by copolymerization of diethylene glycol component and the like does not show properties which polyethylene terephthalate inherently has. This film is also unsatisfactory in terms of hand tearability and twist retention.